Heart disease kills or cripples more people in the prime of life than all other diseases combined. Coronary artery disease is the major form of heart disease and accounts for over 67 percent of all heart disease deaths and heart attacks. Coronary artery disease also accounts for over 70 percent of the $160 billion direct medical costs spent for heart disease in the U.S. in 1994. Arteriosclerosis is the most common cause of this disease, with less common causes including syphilitic and rheumatic arteritis and coronary ostial obstruction associated with aortitis, embolism, periarteritis nodosa, and congenital cardiovascular anomalies. While coronary artery disease is essentially a disease of middle and old age, primarily afflicting men, the disease is being recognized with increasing frequency in women and in the younger age groups. Early detection of this disease would improve patient quality of life and decrease the risk of death as it would provide the opportunity for the effective implementation of low cost, less invasive treatment. However, even though coronary artery disease is a progressive disease it is initially diagnosed by physicians in only 17 percent of cases. The primary means of diagnosing this disease is due to sudden cardiac death (64 percent of cases) and acute heart attack (19 percent of cases) Where coronary artery disease is diagnosed by a physician, it is generally done through the use of costly, invasive technologies, many of which use toxic chemicals or ionizing radiation, and have considerable mortality risk. The diagnostic devices currently used by physicians in a normal physical exam are almost incapable of detecting coronary artery disease at a point where it is almost fully treatable with drugs, exercise and lifestyle modifications.
Coronary artery disease involves a narrowing of one or both of the coronary arteries that supply blood to the myocardium of the heart. This narrowing or constriction of the artery is frequently termed a stenosis. Where a blood vessel is narrowed or partially occluded, tissues normally served by arterial vessels located distal from the point of stenosis can be damaged due to the lack of sufficient blood flow. Where a stenosis occurs in the coronary artery, the result is myocardial ischemia which is an inadequate blood supply to the myocardium. When the coronary artery blood flow is sufficiently reduced, the cardiac muscle is damaged due to the lack of blood supply. Coronary heart disease results when the coronary artery restriction is sufficient to damage the cardiac muscle. Once cardiac damage has occurred the patient may experience thoracic pain known as angina pectoris. Where the damage is more extensive, the patient may experience myocardial infarction, or heart stoppage.
Early detection of coronary artery disease allows for the effective implementation of low cost, low invasion treatment modalities. However, as the coronary artery disease progresses, treatment options become restricted to high cost, high invasion therapies such as percutaneous transluminal coronary angioplasty and coronary artery bypass graft. Early detection is difficult to achieve because prior to significant ischemia becoming manifest, there are few if any symptoms of coronary artery disease. Indeed, in the majority of cases, the primary diagnostic instrument for coronary artery disease is sudden cardiac death.
In patients without symptoms the primary diagnostic instrument for coronary heart disease has been auscultation with the traditional stethoscope. A coronary stenosis generates a sound component that is in the audio frequency range due to turbulent blood flow in the partially occluded coronary arteries. This audio component is not present in a healthy patient. In traditional auscultation, the physician attempts to detect and differentiate the abnormal blood flow sounds that have pathological origin from normal physiological sounds. Turbulent blood flow in a partially occluded coronary artery generates relatively high frequency components from about 200 hertz to about 2000 hertz with unique frequency components in the 400-1200 hertz range. Other heart sounds associated with contraction and normal heart valve operations are generally louder and concentrated in the lower frequency regime from about 10 hertz to about 200 hertz. Differentiation between these normal louder and lower frequency sounds and the abnormal weaker and higher frequency sounds associated with coronary artery disease by auscultation with a traditional stethoscope is very difficult, and often impossible. First, the audio component of coronary stenosis is very weak and heavily contaminated with noise from other patient heart sounds, other normal patient body sounds and external ambient noise in the room. Second, the audio component of the coronary stenosis is heavily attenuated as it passes through the patient's chest and chest wall. Additional factors decreasing the likelihood of coronary artery disease detection through ausculation with a traditional stethoscope included aural degredation of the examiner, temporal variance of the signals, frequency response of the stethoscope and duration of the signal associated with coronary artery disease. The probability of detecting coronary artery disease using auscultation with a traditional stethoscope is exceedingly low. There are only about a half dozen or so published accounts wherein coronary artery disease was detected by way of auscultation and these were only on patients with thin chest walls with the heart lying in contact with the chest wall and the disease present in the anterior coronary arteries.
Coronary cineangiography is another technique and method for the early detection of coronary artery disease. Coronary cineangiography is one of the most definitive procedures of detection and involves injection, by way of an anterior catheter, of a radio-opaque dye into the coronary arteries of the patient, and then monitoring the dye through serial X-ray films. In this manner the physician can detect a narrowing, or stenosis, of the coronary arteries. It has the advantage of providing good identification of the disease progress. However, as an invasive procedure, coronary angiography subjects the patient to high risk and costs and is therefore not suitable for routine medical screening in asymptomatic patients.
Another technique, described in U.S. Pat. No. 5,159,932 to Zanetti et al., attempts to diagnose the cardiac ischemia, or insufficient blood supply to the cardiac muscle, through detection and display of ischemia induced variation in the cardiac motion, which can indicate coronary artery disease. The method of Zanetti, detects cardiac induced compression wave patterns at the patient's chest wall with an inertial detector. Zanetti compares the compression wave patterns and electrocardiogram data for pre-exercise, post-exercise and recovery periods.
Diagnostic methods that are used in conjunction with a "stress test" may prove effective for early detection of coronary artery disease, however, they still require some damage or dysfunction to the heart muscle (although the damage or dysfunction may be temporary) and are not entirely suitable as a physical exam screening technique for patients who are generally asymptomatic for coronary artery disease. The additional expense, time and physician effort involved in a stress test, as well as the patient discomfort, would contraindicate the use of these types of methods in a normal physical exam.
Once cardiac damage has occurred, the damage can be detected through analysis of the patient's cardiac electrical activity under stress with an electrocardiogram. Because electrocardiogram detects damage to the cardiac muscle, and not the cardiac artery disease itself; it does not provide a tool for early cardiac artery disease diagnosis.
A need exists for a non-invasive, low-cost and reliable means for early detection of cardiovascular disease or disorder, allowing for earlier therapeutic intervention.